Oil sand extraction processes are used to liberate and separate bitumen from oil sands so that the bitumen can be further processed to produce synthetic crude oil. Numerous oil sand extraction processes have been developed and commercialized, many of which involve the use of water as a processing medium. Other processes are non-aqueous solvent-based processes. An example of a solvent-based process is described in Canadian Patent Application No. 2,724,806 (Adeyinka et al, published Jun. 30, 2011 and entitled “Process and Systems for Solvent Extraction of Bitumen from Oil Sands). Solvent may be used in both aqueous and non-aqueous processes.
One water-based extraction process is the Clark hot water extraction process (the “Clark Process”). This process typically requires that mined oil sands be conditioned for extraction by being crushed to a desired lump size and then combined with hot (e.g. 95° C.) water and perhaps other agents to form a conditioned slurry of water and crushed oil sands. In the Clark Process, an amount of sodium hydroxide (caustic) may be added to the slurry to increase the slurry pH, which enhances the liberation and separation of bitumen from the oil sands. Other water-based extraction processes may use other temperatures and may include other conditioning agents, which are added to the oil sand slurry, or may operate without conditioning agents. This slurry is first processed in a Primary Separation Cell (PSC), also known as a Primary Separation Vessel (PSV), to extract the bitumen from the slurry.
An overall bitumen extraction process is depicted in FIG. 1. The water and oil sands slurry 100 is separated into three major streams in the PSC 101: bitumen froth 102, middlings 104 and PSC underflow 106. Further processing of each of these streams is explained below. Also shown in FIG. 1, is the solvent 108 added for froth treatment 110, bitumen 112, TSRU (tailings solvent recovery unit) tailings 114, flotation cells 116, recycle bitumen froth 118, fine flotation tailings (FFT) 120, and an external tailings area (ETA) or tailings ponds 122.
Regardless of the type of water-based extraction process employed, the process will typically result in the production of a bitumen froth 102 that requires treatment with a solvent. For example, in the Clark Process, a bitumen froth stream comprises bitumen, fine particulate solids (also referred to as mineral or inorganic solids) and water. Certain processes use naphtha to dilute bitumen froth before separating the product bitumen by centrifugation. These processes are called naphtha froth treatment (NFT) processes. Other processes use a paraffinic solvent, and are called paraffinic froth treatment (PFT) processes, to produce pipelineable bitumen with low levels of solids and water. In the PFT process, a paraffinic solvent (for example, a mixture of iso-pentane and n-pentane) is used to dilute the froth before separating the product, diluted bitumen, by gravity. A portion of the asphaltenes in the bitumen is also rejected by design in the PFT process and this rejection is used to achieve reduced solids and water levels. In both the NFT and the PFT processes, the diluted tailings (comprising water, solids and some hydrocarbon) are separated from the diluted product bitumen.
Solvent is typically recovered from the diluted product bitumen component before the bitumen is delivered to a refining facility for further processing.
One PFT process will now be described further, although variations of the process exist. The PFT process may comprise at least three units: Froth Separation Unit (FSU), Solvent Recovery Unit (SRU) and Tailings Solvent Recovery Unit (TSRU). Two FSUs may be used, as shown in FIG. 2.
With reference to FIG. 2, mixing of solvent with the feed bitumen froth 200 is carried out counter-currently in two stages: FSU-1 and FSU-2, labeled as Froth Separation Unit 1 202 and Froth Separation Unit 2 204. The bitumen froth comprises bitumen, water, and fine solids (also referred to as mineral solids). A typical composition of bitumen froth is about 60 wt % bitumen, 30 wt % water, and 10 wt % solids. The paraffinic solvent is used to dilute the froth before separating the product bitumen by gravity. Examples of paraffinic solvents are pentane or hexane, either used alone or mixed with isomers of pentanes or hexanes, respectively. An example of a paraffinic solvent is a mixture of iso-pentane and n-pentane. In FSU-1 202, the froth 200 is mixed with the solvent-rich oil stream 201 from the second stage (FSU-2) 204. The temperature of FSU-1 202 is maintained at, for instance, about 60° C. to about 80° C., or about 70° C., while the solvent to bitumen (SB) ratio may be from 1.4:1 to 2.2:1 by weight or may be controlled around 1.6:1 by weight for a 60:40 mixture of n-pentane: iso-pentane. The overhead from FSU-1 202 is the diluted bitumen product 205 (also referred to as the hydrocarbon leg) and the bottom stream from FSU-1 202 is the tailings 207 comprising water, solids (inorganics), asphaltenes, and some residual bitumen. The residual bitumen from this bottom stream is further extracted in FSU-2 204 by contacting it with fresh solvent 209, for instance, in a 25 to 30:1 (w/w) SB ratio at, for instance, about 80° C. to about 100° C., or about 90° C. Examples of operating pressures of FSU-1 and FSU-2 are about 550 kPag and 600 kPag, respectively. The solvent-rich oil (overhead) 201 from FSU-2 204 is mixed with the fresh froth feed 200 as mentioned above. The bottom stream from FSU-2 204 is the tailings 211 comprising solids, water, asphaltenes and residual solvent, which is to be recovered in the Tailings Solvent Recovery Unit (TSRU) 206 prior to the disposal of the tailings 213 in tailings ponds. The recovered solvent 218 from TSRU 206 is directed to the solvent storage 210. Solvent from the diluted bitumen overhead stream 205 is recovered in the Solvent Recovery Unit (SRU) 208 and passed as solvent 217 to Solvent Storage 210. Bitumen 215 exiting the SRU 208 is also illustrated. The foregoing is only an example of a PFT process and the values are provided by way of example only. An example of a PFT process is described in Canadian Patent No. 2,587,166 to Sury.
TSRU tailings stream 213 may comprise both coarse and fine solids and is sent for further treatment or disposed in an External Tailings Area (ETA). In general, coarse solids in TSRU tailings settle relatively rapidly while fine solids tend to remain in suspension. Fine solids concentrate to about 30 wt % solids in about two to three years but only very slowly thereafter, particularly in an ETA. This settled material is termed Mature Fine Tailings (MFT) and may also come from other bitumen processing, for instance naphthenic froth treatment.
As depicted in FIG. 1, from the PSC 101, the middlings stream 104, comprising bitumen and about 20-25% solids, is withdrawn and sent to the flotation cells 116 to further recover bitumen. The middlings 104, comprising bitumen, solids and water are processed by bubbling air through the slurry and creating a bitumen froth 118, which is recycled back to the PSC 101. The fine flotation tailings 120 from the flotation cells 116, comprising mostly solids and water, are sent for further treatment or disposed in an ETA.
As depicted in FIG. 1, the PSC underflow 106 from the PSC 101 is sent to an ETA or tailings pond 122. Tailings going into an ETA or tailings pond may include PSC underflow during regular operation and TSRU tailings and FFT during operation upsets. The PSC underflow 106 predominantly comprises coarse sands and has about 55% solids. Coarse solids settle rapidly while fine solids tend to remain in suspension. The fluid fine solids from all streams concentrate to about 30% mass solids in two to three years but only very slowly thereafter. This material, termed Mature Fine Tailings (MFT), is naturally generated over extended periods of time from streams disposed in an ETA.
It is desirable to dewater tailings (slurries) in order to (a) reuse the recovered water, (b) to have a smaller overall volume of slurry, and/or (c) to have a slurry with higher solids content that meets regulations criteria.